1. Field of the Invention
This invention relates generally to a digital computer subsystem for storing data, and more specifically to an open loop, stepper actuated hard disk drive with a low inertia single component actuator arm which positions the read/write heads/flexure assemblies that are each anchored at a single point to the actuator arm, and with a temperature compensation system for maintaining each read/write head over the centerline of the data track.
2. Description of the Prior Art
All computers regardless of size comprise the same basic subsystems: a central processing unit, a means for displaying information, a means for entering information, and a means for storing data. Several means for storing data currently exist including floppy disks, tape drives and hard or fixed disks. Floppy disks have a limited storage capacity. Tape drives are useful for archiving and backing up data, but tape drives are not appropriate for random access applications. The device most commonly used for storing large amounts of data is the hard disk drive.
The hard disk drive has a rotating medium, i.e., a disk, that can be magnetized in a certain pattern, and a floating read/write head. The read/write head creates data patterns on the rotating disk and the read/write head also reads these data patterns. In most hard disk drives several of the disks are mounted above one another, and each of the disks has a read/write head.
In FIGS. 1a and 1b, a prior art open loop stepper actuated hard disk drive 40 is illustrated. The disk drive 40 has two hard disks 25, 26, each of which has a magnetic recording medium on both the top surface 25a, 26a and the bottom surface 25b, 26b. An uppermost flexure 31 holds a read/write head 21 for top surface 25a of first hard disk 25 and a second flexure 32 holds a read/write head 22 for bottom surface 25b of hard disk 25. Similarly, the third flexure (not shown) holds read/write head 23 for top surface 26a of second hard disk 26, while a bottom flexure 34, the fourth flexure, holds a read/write head 24 for bottom surface 26b of hard disk 26. As used herein, a flexure means a flexible arm used to couple a read/write head to an actuator arm.
To read or write data on hard disks 25, 26, heads 21-24 must be positioned such that one of the heads is over a track on the hard disk that is to be used for data storage/retrieval. Since flexures 31-34, which hold heads 21-24, respectively, are all anchored to a single actuator arm 35, as actuator arm 35 is moved, heads 21-24 traverse the tracks on disks 25, 26 of disk drive 40. Accordingly, an electronic circuit (not shown) in disk drive 40 selects one of heads 21-24 for reading or writing in response to commands from a disk controller which interfaces disk drive 40 with a central processing unit.
Actuator arm 35 is moved by means of a stepper motor (not shown) which drives capstan 13 which in turn is coupled to aluminum actuator arm 35 by a band 14. Band 14 is made of stainless steel and has a first portion comprising two parallel strips 14a, 14b that are joined at one end by a metal rectangle 27 having a slot. The slot in rectangle 27 is placed over a tab 28. Tab 28, which is a piece of metal separate from actuator arm 35, is secured to actuator arm 35 by a screw 29. A second portion of band 14 is a single strip 14c which has two holes at an end 38 furthest from capstan 13. A clip 11, welded to strip 14c, is positioned over a hole in strip 14c closest to capstan 13 and a screw 10, which passes through clip 11 and strip 14c, secures the end of band 14 to actuator arm 35.
Parallel strips 14a, 14b of band 14 are joined to strip 14c of band 14 by a solid section in the middle of band 14 which contains a hole. The hole in the center of band 14 is force fit over a pin 12 on capstan 13. Hence, a first end of band 14 is anchored by tab 28 to actuator arm 35 and a second end of band 14 is anchored by screw 10 and clip 11 to actuator arm 35 while the middle of band 14 is wrapped around capstan 13 and attached to capstan 13 by force fitting a hole in band 14 on capstan pin 12. As the stepper motor turns capstan 13, the capstan moves band 14 which in turn moves the actuator arm 35 to which the read/write heads are attached. Accordingly, by precisely controlling the stepper motor, read/write heads 21 to 24 may be positioned over each of the tracks on disks 25, 26.
To assure proper operation of band 14, capstan 13 and actuator arm 35, band 14 is tensioned so that at the lowest operating temperature for disk drive 40 the motion of capstan 13 is uniformly transferred by band 14 to actuator arm 35. However, proper tensioning of band 14 is difficult.
Band 14 is tensioned during assembly of disk drive 40. The apparatus (FIG. 2) used to tension band 14 is difficult to use and disk drive 40 is frequently damaged during assembly. Thus, the production efficiency is directly related to the ability to tension band 14 without damaging the drive.
The tensioning apparatus consists of a spring 50 attached to a line 51 which in turn is attached to a body 52 of the apparatus. An end 52a of tensioning apparatus body 52 fits over end 35a (FIG. 1) of arm 35 so as to prevent movement of arm 35. To tension band 14, spring 50 is attached to hole 37 and then end 52a is placed over end 35a of arm 35. The body 52a is held so that the resulting expansion of spring 50 tensions band 14 and then screw 10 is tightened to maintain the tension. Since the tensioning apparatus passes from hole 37 between disks 25, 26 to the perimeter of drive 40 and then around the perimeter of the drive 40 to base 35a of actuator arm 35, the length of the tensioning apparatus and the close proximity of disks 25, 26 makes tensioning band 14 cumbersome. The apparatus must be held steady so that the surface of the disk 25, 26 is not contacted while affixing the tensioned band to arm 35. If the tensioning apparatus contacts the surface of either disk 25 or disk 26, the disk is very likely to be damaged.
Since hard disk drives are designed to handle large amounts of data, the hard disk drive must have the ability to rapidly access the data and provide the data to the central processing unit. There are several factors which effect the speed with which a hard disk drive accesses data. These factors include the rotational speed of the disk, the density of data storage on the disk, and the time required to position the read/write heads over the track on the disk, i.e. the access time.
For an open loop stepper actuated hard disk drive 40, such as that in FIG. 1a and FIG. 1b, the inertia of the actuator arm 35 is a significant factor in the determination of the access time, because an actuator arm having high inertia requires a longer time period to start or stop the motion of the arm for a given size of the stepper motor. For hard disk drive 40, a typical access time is 60 milliseconds.
In addition to a fast average access time, the open loop, stepper actuated disk drive, has several other performance specification requirements. These include: (a) operational linear and rotational shock and vibration specifications; (b) non-operational linear and rotational shock and vibration specifications; and (c) maintenance of the read/write head at the center line of the data track under varying temperature conditions. The operational specifications assure proper operation of the disk drive during normal use, while the non-operational specifications are intended to prevent damage to the disk drive during shipping.
The inertia of the actuator arm also affects the ability to meet the specifications for linear and rotational shock and vibration. As the inertia of the actuator arm increases, the ability to maintain the specifications decreases because once the arm is set in motion by a shock or vibration, the arm continues to move until the inertia is dissipated, and the range of motion required to dissipate the inertia for a high inertia arm is likely to be greater than the specifications.
Since a typical hard disk has 600 to 800 concentric tracks over a one-inch radius of the disk, the requirement that the open loop stepper actuated disk drive remain at the center line of the data track under different temperature conditions means that either the disk drive must be maintained in a controlled environment wherein temperature fluctuations are minimal or that the disk drive must include means for compensating for different temperature conditions. This is necessary because the components of the disk drive change size with changes in temperature. As the temperature increases, the components expand and as the temperature decreases the components contract. Hence, a change in temperature may move the read/write head off the centerline of the track. If the read/write head is not centered on the track, the data is not accurately read or written. Disk drives, which are typically used with personal computers, are not usually in a controlled environment and accordingly a means for temperature compensation such that the read/write heads remain over the center of the track for a range of temperatures is necessary.
In disk drive 40, illustrated in FIG. 1a and FIG. 1b, precise temperature compensation is limited by the method of attaching the flexure to the actuator arm. Stainless steel flexure 31 is attached to an aluminum plate 15 by two screws 18, 19. The aluminum plate 15 has two guide holes 17, 20, which fit over protrusions on aluminum actuator arm 35, and plate 15 is secured to actuator arm 35 by a screw 16. Flexure 34 is coupled to actuator arm 35 in a similar manner. The other flexures are each attached directly to the aluminum actuator arm 35 by two screws. The thickness of the portions of band 14 are selected so that the thermal expansion of band 14, capstan 13 and actuator arm 35 causes actuator arm 35 to move so as to offset the thermal expansion of aluminum plates 15, disks 25, 26 and flexures 31 to 34 so that the position of read/write heads 21 to 24 relative to disks 25, 26 tends to remain unaffected by temperature changes.
However, actuator arm 35, the aluminum plates, and the flexures expand/contract with temperature changes at different rates because each consists of a different material. Since each stainless steel flexure is anchored at two points to aluminum, the difference in thermal expansion between the flexure and aluminum may cause the flexure to buckle between the anchor points. Also, the numerous components attached to actuator arm 35 in disk drive 40 make it difficult to adequately compensate for temperature variations using a band 14 and actuator arm 35 thermal compensation system. A band and actuator arm system can offset thermal expansion for a single set of continuous boundary conditions, where a set of continuous boundary conditions correspond to the boundary conditions for a single method of affixing a flexure to the actuator arm which do not change abruptly with changes in temperature. However, each of the screws and interfaces between the various components introduce a different boundary condition and hence disk drive 40 may have several sets of boundary conditions.
The boundary conditions for the top flexure 31 and the bottom flexure 34 are different than the boundary conditions for the other two flexures because the two sets of flexures are attached differently to the actuator arm 35. As a result of the different boundary conditions, the thermal expansion of the first set of flexures, flexure 31 and 34, with a first set of boundary conditions may not be the same as the thermal expansion of the other set of flexures with a second set of boundary conditions. Accordingly, the ability to predict thermal expansion is difficult and subject to variations, and the required thermal compensation may be different for each flexure. Hence, in disk drive 40 in FIGS. 1a, 1b, the precision with which the read/write heads can be maintained over the center line of the tracks is limited by the attachment of the flexures to the actuator arm and the inability of band 14 and actuator arm 35 to effectively compensate for the different rates of thermal expansion of the various components caused by the different boundary conditions.
Also, as the temperature changes the boundary conditions may change. For example, if a flexure buckles, then the surface of the flexure is no longer in direct contact with actuator arm and hence the boundary condition changes for the temperature at which the flexure buckles. Therefore, the boundary conditions in the prior art disk drive may change abruptly with a change in temperature.
The aluminum plates, the screws used to attach the aluminum plates to the actuator arm, the screws used to attach the flexures to the aluminum plates, and the screws used to attach flexures directly to the actuator arm, increase the inertia of the assembled actuator arm. As described above, increased inertia reduces both the access time and the ability to meet linear and rotational shock and vibration specifications. Hence, the numerous plates and screws not only make thermal compensation difficult but also directly decrease the performance of the disk drive.